1. Field of the Invention
The present invention relates to an exhaust system of a motorcycle, and more particularly to an exhaust system of a motorcycle which has a V-type engine and an exhaust gas purifying function.
2. Description of Related Art
In a typical exhaust system of a motorcycle with a V-type engine, front exhaust pipes connected with cylinders disposed in front of an engine and rear exhaust pipes connected with cylinders disposed behind the engine are joined to an exhaust muffler via an exhaust chamber.
In this layout, the front and rear exhaust pipes preferably have an optimum length to secure appropriate function of the engine. While the front exhaust pipes sufficiently have the optimum length, the rear exhaust pipes have only a short length to reach the exhaust chamber and therefore cannot easily obtain the same length as that of the front exhaust pipes.
Considering this limitation, JP-A-62-60914 discloses an exhaust system where the length of the rear exhaust pipes is made substantially equal to that of the front exhaust pipes by connecting the rear exhaust pipes to the exhaust chamber in a zigzag line.
In addition to the requirement for sufficient engine performance, there is also a demand for purification of exhaust gas discharged from the exhaust system. In this regard, JP-A-59-99020 proposes a technique of containing catalyst in the exhaust muffler.
FIG. 5 is a cross-sectional plan view of exhaust chamber 101 of JP-A-59-99020. Front exhaust pipes 102a and 102b are connected with a front part of exhaust chamber 101, and rear exhaust pipes 103a and 103b are connected with an upper part of exhaust chamber 101. Rear exhaust pipes 103a and 103b cross each other within exhaust chamber 101 and extend from the cross point. Open ends of front exhaust pipes 102a and 102b and rear exhaust pipes 103a and 103b are directed toward open ends of joint pipes 104a and 104b, which are joined to an exhaust muffler (not shown).
In this structure, rear exhaust pipes 103a and 103b have sufficient length, and high-temperature exhaust gas discharged from front exhaust pipes 102a and 102b and rear exhaust pipes 103a and 103b is injected to joint pipes 104a and 104b such that exhaust gas is introduced to the catalyst contained in the exhaust muffler without lowering the temperature of the exhaust gas. Thus, performance of the engine is secured and purification of the exhaust gas is promoted.
According to JP-A-59-99020, exhaust gas is purified by a catalyst contained in the exhaust muffler. The catalyst comprises ceramics and metal foil (like a honeycomb) coated with noble metals such as platinum or metal fibers, and thus the weight of the catalyst may be one kilogram or more. Location of such a heavy component at a rear position, away from the center of gravity of the vehicle body, deteriorates steering stability.
Additionally, since the rear exhaust pipes are extended within the exhaust chamber to obtain sufficient length, no space for accommodating a catalyst is left within the exhaust chamber. It is therefore difficult to position the catalyst within the exhaust chamber as well as difficult to locate a heavy component of the motorcycle near the center of gravity of the vehicle body.